Add script to run menuconfig properly

This accounts for the fact that running menuconfig properly requires
that all of our compiler environment variables are set correctly.

BUG=None
TEST=```
~/cos/src/scripts/cos/menuconfig.sh
--kernel_tree=../third_party/kernel/v6.18 --arch=x86
--config=lakitu_defconfig
```
and
```
~/cos/src/scripts/cos/menuconfig.sh
--kernel_tree=../third_party/kernel/v6.18 --arch=arm64
--config=lakitu_defconfig
```
RELEASE_NOTE=None

Change-Id: I0d13b536c162ded1c758369781ec4951e3b02258
Reviewed-on: https://cos-review.googlesource.com/c/cos/scripts/+/144904
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Tested-by: Kevin Berry <kpberry@google.com>